I don't think that the pix will allow traffic that was sent on one interface, say the one connected to 4506 device A to be received on another interface, say 4506 device B. The pix ASA algorithm will reject the load-balanced traffic across the different interfaces. That is how pix v6.3 and lower operate. I am not sure about pix v7. What version of pix code are you running? You may find it easier to buy and implement a fail-over pix unit that could connect to the 2nd 4506.
If I find anything out about pix v7 code with regards to this issue, I'll post it here.
